1. Field of the Invention
The present invention relates to a surface acoustic wave filter utilizing a shear horizontal (hereinafter abbreviated as SH) type surface wave, and more particularly, to a longitudinally coupled surface acoustic wave (hereinafter abbreviated as SAW) filter.
2. Description of the Related Art
A longitudinally coupled double-mode SAW filter utilizing a surface wave has been known as one type of various SAW devices. An example of this type of longitudinally coupled SAW filter is shown in FIG. 1.
A SAW filter 51 has a construction in which first and second interdigital transducers (hereinafter abbreviated as IDTs) 53 and 54 are disposed on a rectangular piezoelectric substrate 52. Reflectors 55 and 56 are disposed on both sides of the IDTs 53 and 54 along the propagation direction of a surface wave excited by the IDTs 53 and 54.
In the SAW filter 51, one of the comb-shaped electrodes 53a of the IDT 53 is an input end, and one of the comb-shaped electrodes 54a of the other IDT 54 is an output end. The other comb-shaped electrodes 53b and 54b of the IDTs 53 and 54 are connected to the ground potential.
When an input voltage is applied to the IDT 53, a surface wave is excited and propagates in a direction which is perpendicular to the direction in which an electrode finger of the IDT 53 extends. The surface acoustic wave is then reflected between the reflectors 55 and 56, and becomes a standing wave. An output based on such a standing wave is extracted by the IDT 54. In this case, as a surface wave, a fundamental mode and a high-order mode (asymmetrical mode) are generated. Therefore, the SAW filter 51 operates as a longitudinally coupled double-mode SAW filter.
The conventional longitudinally coupled SAW filter 51 has the problem that the overall dimensions of the filter are excessively large because of the presence of the reflectors 55 and 56. Also, in the SAW filter 51, since an electromechanical coupling coefficient of a piezoelectric substrate 2 is not very large, only a narrow band filter can be achieved.
An edge reflection type longitudinally coupled surface acoustic wave filter utilizing an SH-type surface wave shown in FIG. 2 is disclosed in Japanese Laid-Open Patent Publication No. 9-69751. This filter can allegedly solve the above problems experienced by the filter shown in FIG. 1.
The SAW filter 61 shown in FIG. 2 includes a rectangular piezoelectric substrate 62. The piezoelectric substrate 62 has two opposing edge surfaces 62a and 62b. First and second IDTs 63 and 64 are disposed on the top surface 62c of the piezoelectric substrate 62. The IDTs 63 and 64 have a pair of comb-shaped electrodes 63a and 63b, and 64a and 64b, respectively, and the numbers of pairs of electrode fingers are almost the same.
In order that an SH-type surface wave, such as a BGS (Bleustein-Gulyaev-Shimizu) wave, is excited and reflected between the two opposing edge surfaces 62a and 62b, one of a pair of outermost electrode fingers of the comb-shaped electrode 63a is arranged so as to be flush with the edge surface 62a of the piezoelectric substrate 62. In a similar manner, one of a pair of outermost electrode fingers of the comb-shaped electrode 64b is arranged so as to be flush with the edge surface 62b of the piezoelectric substrate 62. One of the comb-shaped electrodes 63a of the first IDT 63 is an input end, and one of the comb-shaped electrodes 64a of the IDT 64 is an output end, and the comb-shaped electrodes 63b and 64b are connected to a ground potential.
When an input voltage is applied to the IDT 63, a surface wave is excited and propagates in a direction that is perpendicular to the direction in which the electrode fingers of the IDT 63 extends. That is, the surface wave propagates in a direction which connects the two opposing edge surfaces 62a and 62b. This surface wave is reflected by the edge surfaces 62a and 62b, and becomes a standing wave. An output based on this standing wave is extracted from the IDT 64.
Therefore, in the edge reflection-type SAW filter 61 utilizing an SH-type surface wave, since a reflector is not required, a longitudinally coupled double-mode SAW filter has a greatly reduced overall size and a loss caused by the presence of a reflector does not occur. Thus, a surface acoustic wave filter having wider band characteristics can be achieved with this structure.
However, in the SAW filter 61, there is a problem in that spurious responses which are dependent upon the distance between the two opposing edge surfaces 62a and 62b appear over the outside of the passband in the filter characteristics, and, in particular, relatively large spurious responses appear in a frequency region in the vicinity of a passband and outside the passband. This problem is specific to an edge reflection-type SAW filter. The SAW filter 51 shown in FIG. 1 having the reflectors 55 and 56 does not usually suffer from such a problem since the spurious response can be suppressed by adjusting the material, thickness, width of the electrode fingers and number of the electrode fingers of the IDTs and reflectors or selecting an appropriate substrate.